Toyota magnetic vss sensor help

[Toybox]

Hello all,

I'm trying to fit my MPGuino and I've run into a snag.

The injector connection was a piece of cake but the VSS is proving tricky.

The automatic gearbox uses a 2 wire magnetic sensor and the 4 bolts on the propshaft as the trigger. From what I read the sensor produces 5V as the bolts pass across the magnet.

The MPGuino does not seem to like this signal. It works at higher speeds (over 20 - 30 km/h) although it seems a bit erratic.

Any assistance would be much appreciated.

[AndrzejM]

You just need to amplify the signal and then MPGuino will cut it to 5V.

[Toybox]

Great, thanks for the response.

Sorry to be needy, but can you suggest a simple amplification circuit which will work?

[AndrzejM]

Something like this should work just fine.

[kamesama980]

If the VSS in the transmission has 2 wires, it's a VR sensor, not a hall effect sensor. it produces a double sawtooth waveform with it's amplitute proportional to speed and proximity to the tone generating magnetics (in this case, the bolts) which is why the MPGuino doesn't see it at low speeds.

If you have a cable driven speedometer it's totally different and I'm in the process of figuring it out because that's how my truck is and my MPGuino doesn't like it.

Sme of the early ABS equipped cars as they get older, the wheel speed sensors start to lose sensetivity and cut out at low speed causing the ABS to kick on coming to a stop on dry pavement. Yours isn't a poor signal from age, it's just below the threshold of the MPGuino

[meelis11]

I dont know what mpguino you have but it normally expects VSS input to be pulsed 12V.

If yours is 5V max, then you can probably change resistor(s) in VSS input (probably around 50K) to smaller value resistors (5K-10k maybe?)
My point is that instead amplify 5V to 12V you change resistors between VSS input pin and microprocessor (input to atmega can be 0-5V)

[AndrzejM]

Originally Posted by meelis11

I dont know what mpguino you have but it normally expects VSS input to be pulsed 12V.

If yours is 5V max, then you can probably change resistor(s) in VSS input (probably around 50K) to smaller value resistors (5K-10k maybe?)
My point is that instead amplify 5V to 12V you change resistors between VSS input pin and microprocessor (input to atmega can be 0-5V)

Changing resistor won't help if signal is below 5V, there is a Zener diode cutting the signal at this level, and resistor is cutting the current not voltage . So you need to amplify the signal.

[Toybox]

Thanks for the responses guys. Given the complexities of the mag sensor to MPGuino setup, I've decided to go a different direction and fit a reed switch to the back of my speedo, with a NE555 based debounce circuit.

I haven't refitted the dash yet, but early bench testing looks positive.

[AndrzejM]

That's cool! I'm hoping that will work for you flawlessly.

[kamesama980]

Where would you put a reed switch???

Also, what year, make, model car is this. I have access to alldata and the half dozen or so non-cable driven speedo toyotas I've looked at all use 3 wire hall switch VSSs sensing a toothed wheel inside the transmission.

In other news, my cable driven speedo has a reed switch that grounds a 12v line from the ECU. IE 12v square wave.